Many machines, particularly earth working machines, use a hydrostatic drive system to drive the traction wheels or tracks of the machine. The machine speed can be regulated by controlling the displacements of the two hydraulic elements of the hydrostatic drive system.
One common problem with earth moving machines which use hydrostatic or hydro-mechanical transmissions is that the machine can be operated in an engine overspeed condition. An overspeed condition occurs when the machine decelerates quickly or the machine travels down a slope in which there is a reversal of torque in the powertrain such that the engine and drivetrain provide a resistive load to the system. In a hydrostatic transmission, the hydraulic motor will act like a pump and the hydraulic pump will act like a motor. This can put undesirable stresses on the drivetrain and the engine. An overspeed condition can thus cause severe damage to the pump, motor and engine by exceeding the designed operating speed of the component.
One attempt at overcoming this problem is monitoring the engine speed and adjusting the displacements of the pump and motor to prevent the overspeeding condition. However, such systems sometimes react to an overspeed condition too late and thus damage certain powertrain components. Also, while these open-loop control schemes do prevent an overspeed condition, they do not directly control machine deceleration to optimize the energy dissipation provided by an engine friction curve. Such an engine friction curve is shown in FIG. 4 where Engine Torque is graphed against Engine Speed.
Some systems attempt to predict an overspeed condition by calculating the derivative or change of engine speed and react upon this calculated term. However, many times a derivative term can be a noisy parameter which is difficult to use as a feedback signal. Some systems attempt to filter the derivative calculation, but the filter may incorporate a lag which the derivative term was originally trying to remove.
The present invention is directed toward overcoming both of the problems as set forth above.